Fluidic pressure is employed in a variety of applications requiring the application of pressure to a workpiece or load. For example, fluidic pressure can be used to actuate clamps that hold a workpiece securely in place on a piece of industrial equipment. Similarly, clamps used to grasp a load for lifting can be actuated by fluidic pressure. Specific applications include the use of fluid-actuated robot arms in manufacturing plants and hostile work environments, as well as fluid-actuated forklifts used when the weight or bulk of a load makes it inconvenient for human handling. The fluidic pressure developed can be above or below (vacuum) that of the atmosphere and the type of fluid employed can be either hydraulic or pneumatic.
A particular application of interest involves the use of a single set of fluid-actuated clamps to lift loads that can be light and soft or relatively heavy and hard. One of the problems encountered in using a fluid-actuated system in this application is the need to control the pressure applied by the clamps to the different loads. For a relatively soft, light load, a light pressure must be applied to lift the load without damage. A relatively hard, heavy load, however, requires the application of additional clamp pressure to produce a grasp that is firm enough to lift the load. Because of the relatively rigid nature of the load, the additional pressure applied does not damage the load. This level of pressure would, however, tend to crush the relatively soft, light load.
To produce the level of clamp pressure required to lift the load without damage, some adjustment in fluidic pressure must be made for the various loads lifted. The use of a single, pressure limit switch controlling the supply of fluid to the clamps would clearly be inadequate. With the switch adjusted to provide the pressure needed to lift a hard, heavy load, relatively soft, light loads would be crushed. On the other hand, with the switch adjusted to prevent the establishment of pressure sufficient to damage light loads, relatively heavy loads could not be lifted.
One device that seems to address the foregoing problem is that disclosed in U.S. Pat. No. 3,168,203 (Gallistel). The Gallistel apparatus provides an operator with a tactile sense of the pressure exerted by a hydraulic actuator upon a load. In response to the tactile sense experienced, the operator tightens or loosens the manual pressure applied to a handgrip that controls the actuator. In this manner, the pressure applied to the load is correspondingly increased or decreased. While the Gallistel apparatus appears capable of grasping and lifting dissimilar loads without damage, it suffers the distinct disadvantage of requiring feedback from a human operator. In many applications, an automatic system, free from human involvement, is desired. It is this problem to which the present invention is addressed.